1. Field of the Invention
The present invention generally relates to a communication system containing wireless communication terminals and wireless base stations. More specifically, the present invention is directed to such a sensor network system that the above-described wireless communication terminals correspond to compact semiconductor chips on which sensors have been mounted.
2. Description of the Related Art
Research capable of realizing ubiquitous society where all of subjects can be connected via networks to each other has been actively developed. Compact-sized IC chips having wireless communication functions known as non-contact IC cards and RFIDs have been rapidly popularized. These compact-sized IC chips have been widely utilized in various application fields such as product distribution managements, production managements, and electronic money systems. Also, since various sorts of sensor functions are mounted on such IC chips, such trials as to monitoring of production lines, home security, and environmental measurements may be realized. Networks and services which are constructed of such sensor chips (IC chips equipped with sensors) are referred to as “sensor networks”, while “sensor networks” can be expected as techniques capable of acquiring real-time information of actual fields into networks, and capable of establishing information basis of ubiquitous society.
As the technical specifications related to the near distance wireless communications of the non-contact IC cards which have been widely used, the international standard specification of ISO/IEL IS 18092 has been defined.
Presently available sensor networks are still under developing and applying stages where various researching institutes and various makers are developing prototype networks and products which are exclusively and independently designed for these owners. Since these sensor chips have been designed based upon the specific individual communication specifications thereof, there is no compatibility as to a physical layer and other layers higher than, or equal to a MAC layer. In most products, local IDs having shorter bit lengths (for instance, 8 bits and 16 bits) are employed so as to identify the respective sensor chips from each other. The exclusive characteristics of these local IDs are guaranteed only in the relative maker, or only in a specific system. As a result, such sensor chips which are manufactured by different makers cannot be communicated with each other. Otherwise, even when sensor chips are manufactured by the same maker, there are some cases that such sensor chips delivered to different users cannot be communicated with each other. Up to now, since sensor networks have not yet been constructed in a large scale, there is substantially no serious problem. However, in a near future, such a technical idea may be probably expected while sensor network systems will be widely utilized and various sorts of technical specifications are desirably standardized. That is, sensor chips which are purchased from a plurality of makers may be commonly used within the same system, and systems which have been separately established may be linked to each other. In order to realize these expectations, general-purpose architecture is necessarily required which is capable of acquiring data sensed by sensor chips, while this architecture does not depend upon a technical specification provided by a specific maker.
In non-contact IC card fields and RFID fields, in order to exclusively identify each chip from these chips, there is such a move that global IDs having exclusive characteristics in the world are being established. For instance, “Ubiquitous ID” having 128 bits has been proposed from the ubiquitous center of the Japanese business groups, whereas “EPC (Electronic Product Code)” having either 64 bits or 96 bits has been proposed from the EPC global of US business groups. These moves will also be applied to sensor networks, and IDs having exclusive characteristic in the world will be necessarily applied to respective sensor chips in near future.
Generally speaking, a passive power supplying system is known as a popular power supplying system in non-contact IC cards and RFIDs, in which electromagnetic waves radiated from readers are used as energy sources so as to operate these cards. On the other hand, since sensor chips for constituting sensor networks are arranged in a wide area, and perform sensing operations and data transmission in an autonomous manner, such an active power supplying system is known as a general-purpose power supplying system in which batteries, or power generating apparatus are built in the sensor chips. Concretely speaking, this active power supplying system employs a primary battery (dry battery), a secondary battery (rechargeable battery), a fuel cell, solar light power generating apparatus, very small vibration power generating apparatus, and the like.
When the following installation ideas may be conceived in which sensor chips are attached to persons and articles and a large number of sensor chips are distributed in an environment, these sensor chips must be made as small as possible. In order to make sensor chips compact, active power supplying apparatus thereof must be made compact. However, this implies reductions of power supply capacities. On the other hand, the respective sensor chips must be driven for long time in order to improve practical utilizations. As a consequence, power consumption of these sensor chips must be necessarily lowered.
The most portion of the power consumption by these sensor chips is consumed in wireless communications, for instance, more than 90% of entire power is consumed. Therefore, in order to reduce power consumption of these sensor chips, wireless communications must be carried out in low power consumption. As one of power reducing aspects, reductions of a total amount of data to be transmitted/received may become important. As previously explained, such an idea for applying global IDs having exclusive characteristics in the world even to these sensor chips may be necessarily expected. Under this condition, bit lengths of measurement data which are wanted to be transmitted typically become several tens of bits. If the global IDs having the long bit lengths are directly used under this environment, then actual communication amounts may be unnecessarily increased, as compared with communication amounts of data.
For instance, in such a case that a global ID having 128 bits is directly used in a header of a data packet during communication operation, if a protocol designates both a transmission source and a transmission destination, then overhead of 256 bits is produced only by employing this protocol. On the other hand, sizes of measurement data may be determined based upon sorts of sensors and measuring objects. When use of temperature data is conceived, a typical bit length of this temperature data may become 16 bits, or 32 bits in maximum. In such usage that real-time characteristics are required to some extent, sensor chips are required to transmit measurement data every time a measuring operation is performed one time. At this time, it is ineffective to give the overhead of 256 bits to the transmission of the measurement data. In conventional wire type communications and conventional wireless type communications, such an overhead never causes a serious problem, while the conventional wireless type communications are established under such an initial condition that there is a relatively large margin in electric power, e.g., wireless LANs and Bluetooth®. However, in sensor chips established under such an initial condition that wireless communications are carried out in low speeds and low power consumption, overhead having 200 bits, or more bits per 1 packet may give seriously adverse influences to communication performance.
As previously explained, among the local IDs which have been employed in the conventional sensor chips, these local IDs merely own the exclusive characteristics only in the relevant maker, or in the relevant sensor chip system. In communications made outside systems, there is no general-purpose method capable of exclusively specifying individual sensor chips from each other. On the other hand, in such a case that the global IDs which are expected to become essential in future are directly employed so as to perform a wireless communication, a ratio of overhead occupied in communication data of sensor chips is increased, so that communication ranges and power consumption are wasted.